Jeremy Hobson: There are reports that 600,000 Apple computers around the world have been infected with a virus. The company has released a fix to protect customers. The news is especially surprising because Apple computers have long been considered all but immune to viruses.

Marketplace's David Gura reports.

David Gura: For years, Apple told its customers their computers are less susceptible to viruses.

Pat O'Shea teaches computer engineering at the University of Maryland and he uses an Apple.

Pat O'Shea: I thought everything was cool. Didn't have to worry. You know, we were secure. Steve Jobs had created this wonderful cocoon for us. And this is a big wake-up call.

Several companies that make anti-virus software say a bot targeted Apple users. That kind of virus has plagued PCs for years. Now, cyber security expert Brian Krebs says Apple users have to worry, and they need to be more vigilant.

Brian Krebs: It really doesn't matter what kind of system you're using. If you're giving the bad guys an easy opportunity to take control of your system, they'll take it.

Krebs says hackers have had a harder time designing malicious software, or malware, for Apple's operating system, but that doesn't explain everything.

Krebs: If malware has left the Mac folks alone, it's because it has not been that big of a player.

For a long time, Apple only controlled a small slice of the computer market. And over these last few years, that's started to change.